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<Title>mem_fun_t&lt;Result, X&gt;</Title>
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<H1>mem_fun_t&lt;Result, X&gt;</H1>

<Table CellPadding=0 CellSpacing=0 width=100%>
<TR>
<TD Align=left><Img src = "functors.gif" Alt=""   WIDTH = "194"  HEIGHT = "38" ></TD>
<TD Align=right><Img src = "type.gif" Alt=""   WIDTH = "194"  HEIGHT = "39" ></TD>
</TR>
<TR>
<TD Align=left><Img src = "adaptors.gif" Alt=""   WIDTH = "194"  HEIGHT = "38" ></TD>
<TD Align=right></TD>
</TR>
<TR>
<TD Align=left VAlign=top><b>Categories</b>: functors, adaptors</TD>
<TD Align=right VAlign=top><b>Component type</b>: type</TD>
</TR>
</Table>

<h3>Description</h3>
<tt>Mem_fun_t</tt> is an adaptor for member functions.  If 
<tt>X</tt> is some class with a member function <tt>Result X::f()</tt> (that is, a 
member function that takes no arguments and that returns a value of
type <tt>Result</tt> <A href="#1">[1]</A>), then a <tt>mem_fun_t&lt;Result, X&gt;</tt> is a <A href="functors.html">function object</A>
adaptor that makes it possible to call <tt>f()</tt> as if it were an 
ordinary function instead of a member function.
<P>
<tt>Mem_fun_t&lt;Result, X&gt;</tt>'s constructor takes a pointer to one of 
<tt>X</tt>'s member functions.  Then, like all function objects, 
<tt>mem_fun_t</tt> has an <tt>operator()</tt> that allows the <tt>mem_fun_t</tt>
to be invoked with ordinary function call syntax.  In this case,
<tt>mem_fun_t</tt>'s <tt>operator()</tt> takes an argument of type <tt>X*</tt>.  
<P>
If <tt>F</tt> is a <tt>mem_fun_t</tt> that was constructed to use the member
function <tt>X::f</tt>, and if <tt>x</tt> is a pointer of type <tt>X*</tt>, then the
expression <tt>F(x)</tt> is equivalent to the expression <tt>x-&gt;f()</tt>.  The
difference is simply that <tt>F</tt> can be passed to STL algorithms whose
arguments must be function objects.
<P>
<tt>Mem_fun_t</tt> is one of a family of member function adaptors.  These 
adaptors are useful if you want to combine generic programming
with inheritance and polymorphism, since, in C++, polymorphism
involves calling member functions through pointers or references.
<P>
As with many other adaptors, it is usually inconvenient to use 
<tt>mem_fun_t</tt>'s constructor directly.  It is usually better to use
the helper function <tt>mem_fun</tt> instead.
<h3>Example</h3>
<pre>
struct B {
  virtual void print() = 0;
};

struct D1 : public B {
  void print() { cout &lt;&lt; &quot;I'm a D1&quot; &lt;&lt; endl; }
};

struct D2 : public B {
  void print() { cout &lt;&lt; &quot;I'm a D2&quot; &lt;&lt; endl; }
};

int main()
{
  <A href="Vector.html">vector</A>&lt;B*&gt; V;

  V.push_back(new D1);
  V.push_back(new D2);
  V.push_back(new D2);
  V.push_back(new D1);

  <A href="for_each.html">for_each</A>(V.begin(), V.end(), mem_fun(&amp;B::print));
}
</pre>
<h3>Definition</h3>
Defined in the standard header <A href="functional">functional</A>, and in the nonstandard
backward-compatibility header <A href="function.h">function.h</A>.
<h3>Template parameters</h3>
<Table border>
<TR>
<TH>
Parameter
</TH>
<TH>
Description
</TH>
<TH>
Default
</TH>
</TR>
<TR>
<TD VAlign=top>
<tt>Result</tt>
</TD>
<TD VAlign=top>
The member function's return type.
</TD>
<TD VAlign=top>
&nbsp;
</TD>
</TR>
<TR>
<TD VAlign=top>
<tt>X</tt>
</TD>
<TD VAlign=top>
The class whose member function the <tt>mem_fun_t</tt> invokes.
</TD>
<TD VAlign=top>
&nbsp;
</TD>
</tr>
</table>
<h3>Model of</h3>
<A href="AdaptableUnaryFunction.html">Adaptable Unary Function</A>
<h3>Type requirements</h3>
<UL>
<LI>
<tt>X</tt> has at least one member function that takes no arguments
   and that returns a value of type <tt>Result</tt>. <A href="#1">[1]</A>
</UL>
<h3>Public base classes</h3>
<tt><A href="unary_function.html">unary_function</A>&lt;X*, Result&gt;</tt>
<h3>Members</h3>
<Table border>
<TR>
<TH>
Member
</TH>
<TH>
Where defined
</TH>
<TH>
Description
</TH>
</TR>
<TR>
<TD VAlign=top>
<tt>argument_type</tt>
</TD>
<TD VAlign=top>
 <A href="AdaptableUnaryFunction.html">Adaptable Unary Function</A>
</TD>
<TD VAlign=top>
The type of the argument: <tt>X*</tt>
</TD>
</TR>
<TR>
<TD VAlign=top>
<tt>result_type</tt>
</TD>
<TD VAlign=top>
 <A href="AdaptableUnaryFunction.html">Adaptable Unary Function</A>
</TD>
<TD VAlign=top>
The type of the result: <tt>Result</tt>
</TD>
</TR>
<TR>
<TD VAlign=top>
<pre>
Result operator()(X* x) const
</pre>
</TD>
<TD VAlign=top>
 <A href="UnaryFunction.html">Unary Function</A>
</TD>
<TD VAlign=top>
Function call operator.  Invokes <tt>x-&gt;f()</tt>, where <tt>f</tt> is
   the member function that was passed to the constructor.
</TD>
</TR>
<TR>
<TD VAlign=top>
<pre>
explicit mem_fun_t(Result (X::*f)())
</pre>
</TD>
<TD VAlign=top>
<tt>mem_fun_t</tt>
</TD>
<TD VAlign=top>
See below.
</TD>
</TR>
<TR>
<TD VAlign=top>
<pre>
template &lt;class Result, class X&gt;
mem_fun_t&lt;Result, X&gt;
mem_fun(Result (X::*f)());
</pre>
</TD>
<TD VAlign=top>
<tt>mem_fun_t</tt>
</TD>
<TD VAlign=top>
See below.
</TD>
</tr>
</table>
<h3>New members</h3>
These members are not defined in the 
<A href="AdaptableUnaryFunction.html">Adaptable Unary Function</A>
requirements, but are specific to 
<tt>mem_fun_t</tt>.
<Table border>
<TR>
<TH>
Member
</TH>
<TH>
Description
</TH>
</TR>
<TR>
<TD VAlign=top>
<pre>
explicit mem_fun_t(Result (X::*f)())
</pre>
</TD>
<TD VAlign=top>
The constructor.  Creates a <tt>mem_fun_t</tt> that calls the member
   function <tt>f</tt>.
</TD>
</TR>
<TR>
<TD VAlign=top>
<pre>
template &lt;class Result, class X&gt;
mem_fun_t&lt;Result, X&gt;
mem_fun(Result (X::*f)());
</pre>
</TD>
<TD VAlign=top>
If <tt>f</tt> if of type <tt>Result (X::*)</tt> then <tt>mem_fun(f)</tt> is the
same as <tt>mem_fun_t&lt;Result, X&gt;(f)</tt>, but is more convenient.  This is a
global function, not a member function.
</TD>
</tr>
</table>
<h3>Notes</h3>
<P><A name="1">[1]</A>
The type <tt>Result</tt> is permitted to be <tt>void</tt>.  That is, this
adaptor may be used for functions that return no value.  However, this
presents implementation difficulties.  According to the draft C++
standard, it is possible to return from a <tt>void</tt> function by writing
<tt>return void</tt> instead of just <tt>return</tt>.  At present, however (early
1998), very few compilers support that feature.  As a substitute,
then, <tt>mem_fun_t</tt> uses partial specialization to support <tt>void</tt> member
functions.  If your compiler has not implemented partial
specialization, then you will not be able to use <tt>mem_fun_t</tt> with
member functions whose return type is <tt>void</tt>.
<h3>See also</h3>
<tt><A href="mem_fun_ref_t.html">mem_fun_ref_t</A></tt>, <tt><A href="mem_fun1_t.html">mem_fun1_t</A></tt>, <tt><A href="mem_fun1_ref_t.html">mem_fun1_ref_t</A></tt>

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